1. Field of the Invention
This invention relates to a dot printer having a needle which is driven to print combinations of dots each defining a particular letter or figure. More particularly, it relates to the head of any such dot printer.
2. Description of the Prior Art
There is known a dot printer in which armatures are actuated upon energization of coils to strike needles against a platen for printing dots. The armatures are driven by a mechanism which is shown by way of example in FIGS. 1 and 2. Each of the coils 3 is disposed about one of a plurality of cores 2 each forming an integral part of a yoke 1. Each of the armatures 5 is supported rotatably about a pivot 6 to cause the needle 4 to strike against the platen when the coil 3 is energized. Each armature 5 has a pair of recesses 8. A guide 7 is provided between every two adjoining armatures 5 and has one end disposed in one of the recesses 8 of one armature, while the other end of the guide 7 is disposed in one of the recesses 8 of the other armature. The guides 7 are located in a plane facing the yokes 1. If the coils 3 are energized, the armatures 5 are attracted toward the cores 2 to drive the needles 4. A magnetic flux travels from the cores 2 to the yokes 1 through the armatures 5 and from the yokes 1 to the cores 2. In order to perform effective printing, it is necessary to increase the attracting force of the cores 2 by applying a small magnetomotive force. This necessitates an increase in the surface area S of the region in which each armature 5 faces the corresponding yoke 1. This in turn requires an increase in the radial width 1.sub.3 of the yoke 1. The distance between the pivot 6 and the center of the core 2 is shown at 1.sub.1, and the distance between the pivot 6 and the free end of the armature 5 at which it hits the needle 4 is shown at 1.sub.2. If the distance 1.sub.3 is increased, it is necessary to increase the distance 1.sub.1 resulting in the necessity for an increased air gap G to maintain an appropriate stroke A.sub.s for the movement of the free end of the armature 5. An increase in the air gap G, however, results in the failure of the core 2 to produce a satisfactorily large attracting force. Moreover, an increase in the distance 1.sub.1 means a lower lever ratio 1.sub.2 /1.sub.1 and an increase in the equivalent mass of the armature 5. This disables fast printing and gives rise to an increase in power consumption. These problems are worsened by the presence of the recesses 8 in the plane facing the yoke 1. If the recesses 8 are taken into account, an increase in the surface area S calls for a greater increase in the distances 1.sub.3 and 1.sub.1 and gives rise to a greater increase in the air gap G.